Relaxation oscillator with plural constant current sources

ABSTRACT

In an oscillatory signal generator circuit exceptional frequency stability is insured by the use of interrelated constant current sources (520, 521, 522) that provide an insensitivity to supply voltage variations over a wide range. Two of the current sources determine the duty cycle by controlling the charge and discharge current of a capacitor (C) in the circuit. A third current source determines the voltage swing on the capacitor. By varying the current output from the third current source, the generator circuit provides a variable frequency signal.

TECHNICAL FIELD

This invention relates to oscillator circuits and, more particularly, tooscillatory signal generators employing constant current sources forproviding an insensitivity to supply voltage variations and a frequencyshifting capability.

BACKGROUND OF THE INVENTION

The use of the conventional electromechanical bell ringer has been theprimary signaling device used in telephones for a great number of years.In recent years, however, tone ringers considered to be more desirablehave been replacing the electromechanical bell ringer due to advances inelectronic technology. The tone ringers provide a signal generallyconsidered substantially more pleasing to the average ear than thejangle of an electromechanical bell ringer. Also, space requirementsthrough use of tone ringers are minimized. These tone ringers compriseelectronic circuitry that responds to conventional low frequency powerringing signals on a telephone line, and also a tone generatingtransducer that provides an alerting signal to a telephone subscriber.An example of such a tone ringer is disclosed in U.S. Pat. No.3,740,490, issued to R. F. McAlonie et al., on June 19, 1973.

Telephone ringers have to operate over a loop whose length can varyconsiderably. Although more efficient than electromechanical bellringers, tone ringers of the prior art are optimized to operate overloops not exceeding a certain length since the ringers have anoperational voltage range. Telephone extensions with ringers increasethe load presented to the ringing voltage resulting in a furtherdecrease in the available voltage. Thus, the value of the currentavailable to the tone ringer is a critical, sensitive factor,particularly so in those instances where multiple sets are terminated ona line. It is desirable, therefore, to provide a tone ringer whichallows for efficient operation over a wide range of loop lengths andmultiple extensions being terminated on the telephone line.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a tone ringer towhich power is provided through use of constant current generators foroptimum performance over a wide range of loop lengths and supply voltagelevels. These current generators provide power to an oscillatorygenerator means within the tone ringer ensuring frequency stability ofthe generator means and an insensitivity to supply voltage variationsover a wide range.

Transistors biased with fixed resistors and unbypassed emitters serve asconstant current generators supplying a fixed value of currentindependent of the supply voltage through a wide operational range. Thecurrent value is established by the base voltage and the emitterresistor, and a high impedance is presented at the collector. Connectionof two such generators, an NPN transistor and a PNP transistor, so thatthey are complementary disposed across a line facilitates driving of theoscillatory signal generator means and other tone ringer circuitcomponents with a constant current and at the same time provides a highimpedance to both sides of the circuit components.

According to the present invention the tone ringer circuit comprisessensing means for distinguishing between valid ringing signals andunwanted transients, oscillatory generator means for energizing anoutput transducer through output circuit means, frequency determiningmeans for varying the frequency of the oscillatory generator means andpower conversion means for deriving ringing power from the input ringingsignal. Organization of the circuit is such that implementation in theform of an integrated circuit is accomplished with a minimum number ofexternal components.

The oscillatory generator means determines the frequency and duty cycleof the signal which drives the transducer. Power for the oscillatorygenerator is derived through constant current generators from the inputringing signal. Amplitude modulation of the signal to the transducer isalso provided by the input ringing signal. The oscillatory generatormeans has a resistor-capacitor combination which sets the frequency ofthe signal generated while the duty cycle is internally set by the ratioof two of the constant current generators. The fundamental frequency ofthe oscillator is changeable by adjusting the resistor or capacitorvalues while maintaining the same duty cycle.

To provide a distinctive ringing capability in accordance with theinvention, in one operating condition the fundamental frequency of theoscillatory generator means is altered by the frequency determiningmeans. The fundamental frequency is altered by a fractional relationshipsuch as a 5/4 ratio for the negative half-cycle of the input ringingvoltage. This provides the option of having a frequency-shift typeoutput signal in addition to the standard amplitude-modulated outputsignal. The duty cycle remains unchanged during production of thefrequency-shift signal.

BRIEF DESCRIPTION OF THE DRAWING

This invention will be more fully comprehended from the followingdescription and the accompanying drawing in which:

FIG. 1 is a block diagram of an electronic tone ringer showing the majorfunctional circuit components of the ringer and their generalinterconnection with each other in accordance with the presentinvention;

FIGS. 2 and 3 present a schematic diagram showing the detailed circuitryof an embodiment of the electronic tone ringer in accordance with theinvention;

FIG. 4 illustrates the spacial arrangement of FIGS. 2 and 3; and

FIG. 5 is an oscillatory signal generator circuit in accordance with theinvention.

DETAILED DESCRIPTION

FIG. 1 is a functional block representation of a tone ringer operativein accordance with the principles of the invention. As shown, the ringercomprises power conversion means 200 connected to a pair of input lines101 and 102 over which incoming ringing signals are received. The powerconversion means 200 serve to rectify the incoming ringing signal andprovide a source of power to the remainder of the tone generatingcircuitry. Voltage limiting circuitry is also included to limit themaximum voltage that is developed across the remainder of the circuitry.In addition, the power conversion means 200 provides a source of voltageover line 103 to drive the electroacoustic transducer 300.

Connected across the output lines 104 and 105 of power conversion means200 is sensing means 400 which determine the presence of a valid inputringing signal. Discrimination between valid ringing signals and dialpulses or switchhook transients is determined by the persistence of theinput signal voltage above a reference voltage level.

Also connected across lines 104 and 105 is oscillatory generator means500 which produces the signal that drives output circuit means 700. Thefrequency at which the oscillatory generator means 500 operates iscontrolled by frequency determining means 600. In one operatingcondition the frequency determining means allows the oscillatorygenerator means to operate at a fundamental frequency and this frequencyis coupled to the transducer via an output circuit means 700. In asecond operating condition the frequency determining means 600 willcause the oscillatory generator means 500 to provide a frequency-shiftedoutput signal which is applied through output circuit means 700 to thetransducer 300. Included in the output circuit means 700 is a latchwhich, when enabled by the sensing means 400, is switched on and off ata rate determined by the oscillatory generator means 500. When activatedboth by the sensing means 400 and oscillatory generator means 500, theoutput circuit means 700 provides a path for current to flow from thetransducer over line 106 to line 105.

Referring now to FIG. 2, there is shown a schematic diagram of thedetailed circuitry of the tone ringer of FIG. 1. The input ringingsignal appearing on lines 101 and 102 is coupled into the powerconversion means 200 to a diode bridge consisting of diodes 201 through204 and also diode 205. Coupled to diode 205 is an electroacoustictransducer 300 and an output circuit means 700 which will be discussedlater. One output from the diode bridge is connected to a diode 206which in combination with a capacitor 207 couples power to the toneringer circuit components. The diode bridge output also has a resistordivider network comprising resistors 208, 209 and 210 attached thereto.One tap on this resistor divider network indicated as node 10 leads to aDarlington transistor configuration 211 and 212 which feeds a voltageregulator formed by transistors 213, 214 and 215. Transistors 213 and215 operate as Zener diodes by virtue of the reverse breakdown of theirbase emitter junctions. These transistors serve several functions. Inconjunction with resistors 208, 209 and 210 and transistors 211 and 212,they limit the maximum voltage at node 11 to a specific level. Thislimit provides a safe margin under the maximum allowable voltage for thestandard buried collector technology used in making the tone ringerintegrated circuit. Secondly, at node 12 they provide a voltage which isused as an input to the sensing means 400, also to be discussed later.Finally, transistor 215 provides a voltage reference used to determinethe current in all of the current sources in the tone ringer circuit.

Established by transistor 215, the voltage at node 13 sets up a currentwhich is determined by resistors 216 through 220 and the base-emitterdrops of transistors 221, 222 and 223. The emitter current of transitor221 (minus its negligible base current) is the emitter current oftransistor 224 (neglecting the small base currents of transistors 224and 225). Transistors 224, 225 and 226 and in the sensing means 400transistors 401, 402 and 403 then form a current mirror where theemitter current of transistor 224 is reflected or ratioed in theemitters of the transistors of the mirror according to the value of theresistor in their respective emitter leads. Thus the currents in theemitter leads of transistors 226, 402 and 403 are twice the current inthe emitter of transistor 224 because of their respective emitterresistors 227, 404 and 405 being half the value of resistor 228 whilethe emitter current of transistor 401, by virtue of resistor 406 beingtwo times the value of resistor 228, is half the current in transistor224. It should be noted that the number of equal area collectors foreach transistor is shown by its number of collector leads. Transistor225 is used as a helper transistor to supply the base drive for thecurrent mirror. Because transistor 226 has a split collector with equalareas, the collectors, designated "a" and "b", will each have half thetotal emitter current flowing therein. Moreover, the emitter current oftransistor 401 is divided into five equal parts via its five collectorswith three-fifths of the current being shunted to the common node andone-fifth to each of nodes 14 and 15.

The emitter current of transistor 407 is divided into five equal parts,four of those parts being shunted to common and the remaining partcoupled to the collector of transitor 408. This transistor along withtransitors 409. and 410 form another current mirror with transistor 411serving as the helper transistor. Transistor 412 is used to equalize thebase currents of transistors 411 and 413 so as to compensate, via thecollector current of transistor 410, for the base current of transistor413. This current adds significantly to the discharge current ofcapacitor 414 if the gain of the transistors should be low. The emittercurrents of transistors 409, 410 and 412 have specific values determinedby the relative size of their emitter junctions (indicated by the numberof emitter leads) and the values of the resistors in their emitters.

Collector b of transistor 401 is tied to the emitters of a transistordifferential pair, 415 and 416, which form a comparator with one of thepair always turned on. When transistor 416 is on, the current fromcollector b of transistor 401 is shunted to common while when transistor415 is on, four-fifths of the collector 6 current of transistor 401 isshunted to common and one-fifth of the current flows into node 16 andserves to charge capacitor 414.

The collector current of transistor 224 is essentially the emittercurrent of transitor 223 (neglecting base currents). Transistor 223forms a current mirror with transistor 417 on FIG. 2 and transistors501, 601, 502 and 503 on FIG. 3. Transistor 222 is used as the helpertransistor for this current mirror.

Operation of the sensor means depends on the value of the voltage atnode 12 and the voltage on capacitor 414. As soon as an input voltage ofsufficient magnitude is applied, transistor 215 starts regulating andthe current sources are powered. Transistor 215 is held in regulation bythe current in collector a of transistor 226. Constant voltages are thenset up along the resistor string 216 through 219 which are tapped asvoltage references. The reference at node 17 is used in the oscillatorygenerator means 500, to be discussed later, and the reference at node18, when connects to transistor 418, sets the minimum voltage thatcapacitor 414 is allowed to charge to. Node 19 furnishes a referencevoltage to the base of transistor 419. Shifted by the current incollector b of transistor 226 through resistor 420, transistor 419provides a voltage on the base of transistor 415 when transistor 433 isoff. Transistor 416 has its base tied to node 12 which is a ratio of thefull wave rectified input voltage.

As previously indicated, the transistor differential pair 415 and 416control the charging of capacitor 414. When the voltage at node 12 islower than the reference on the base of transistor 415, then transistor416 is on, 415 is off, and the current in collector b of transistor 401flows to common. During this time, capacitor 414 is discharged by thecollector current of transistor 410. When the voltage on node 12 ishigher than the voltage on the base of transistor 415, however,transistor 415 will be on, 416 will be off, and the current in collectora of transistor 415, minus the current of transistor 410, is used tocharge capacitor 414. The voltage across capacitor 414 will rise in atriangular wave fashion dictated by the amount of time that the voltageat node 12 is greater than that at the base of transistor 415. By usinglow charge and discharge currents, capacitor 414 is a relatively smalland inexpensive capacitor.

Transistors 409, 413, 417 and 421 through 424 form a unity gainamplifier to buffer the high impedance node 16 to the lower impedancebase of transistor 425. A differential pair is formed by transistors 425and 426. The base of transistor 426 is tied to a reference voltagedetermined by the constant collector current of transistor 403, avoltage level shifter transistor 427, resistors 428, 429, 430 and 431,and the current in transistor 425.

When the voltage on capacitor 414 is below that on the base oftransistor 426, transistor 425 is on, transistor 426 is off, and thecollector current of transistor 402 flows through transistor 425 andresistor 430 to ground. This additional current through resistor 430serves to raise the voltage on the base of transitor 426 and providescircuit hysteresis as follows. When capacitor 414 charges sufficientlyto raise the voltage at the base of transistor 425 to approximately thatat the base of transistor 426, the transistors switch states andtransistor 425 is off and transistor 426 is on. At this point threethings occur simultaneously. First, three-fourths of the emitter currentof transistor 426 flows to the output circuit means 700, to be discussedlater, turning it on and allowing ringing to begin, since at this pointthe oscillatory generator means 500 is producing a signal. Secondly,one-fourth of the emitter current of transistor 426 flows to thecollector and base of transistor 432, which forms a current mirror withtransistor 433. The ratio of the emitter resistors 434 and 435,respectively, for transistors 432 and 433 is such that the collector oftransistor 433 develops a current which, subtracted from the current incollector b of transistor 226, leaves a low current flowing throughresistor 420. This current reduction serves to reduce the referencevoltage at the base of transistor 415 to a level that ensures that theringer stays on once it starts ringing. Such a reduction is necessary,because once the ringer starts operating, the transducer load causes theinput ringing voltage to drop on its positive half cycle. Finally andalso simultaneously, the collector current flowing in transistor 425through the resistor 430 ceases reducing the reference voltage at thebase of transistor 426 to a level that ensures that a ringer stays on.Thus, once a valid ringing signal is recognized, a positive latch isassured by the reference level shifts. Dial pulse and other transientsignals are not detected by this means because their levels and theirdurations above the reference voltage at the base of trasistor 415 arerespectively less than or are shorter than the input ringing voltage.Thus capacitor 414 is not charged sufficiently to exceed the referencevoltage of node 30 and ringing is inhibited for these signals.

Referring now to FIG. 3, there is seen a schematic representation of theoscillatory generator means 500, frequency determining means 600, andthe output circuit means 700 of the tone ringer circuit. The oscillatorygenerator means 500 functions similarly to the sensing means 400 in thatit uses the charge and discharge of a capacitor by constant currentsources connected in a complementary configuration to set its frequencyand duty cycle. Here capacitor 505 responds to the charging anddischarging current sources which are transistors 504 and 503,respectively. Depending on which transistor, 506 or 507, of thedifferential pair is turned on, capacitor 505 is either charged ordischarged. As previously indicated, transistors 501, 502 and 503 arepart of the current mirrors formed by transistor 223 shown on FIG. 2.

Transistors 502 and 503 have the same current flowing in theircollectors at the time that all the current sources are powered up. Atthis point the reference voltage at node 17 shown in FIG. 2 minus theV_(be) of transistor 508 plus the voltage drop across resistor 514appears at the base of transistor 506. Initially, capacitor 505 isdischarged and, therefore, the base of transistor 507 is below that oftransistor 506 causing transistor 507 to be off and transistor 506 to beon. As a result, collector current in transistor 502 flows throughtransistor 506 and becomes essentially the emitter current of transistor509. Transistor 509 then forms a current mirror with transistors 602,510, 504 and 511. Transistors 512 and 513 form a Darlington pair helpertransistor to provide the base current for the current mirror.

The current through transistor 510 flows through resistor 514 andtransistor 501 to ground. The resultant voltage drop across resistor 514adds to the voltage at the emitter of transistor 508 raising the voltagelevel at the base of transistor 506. At the same time, the collectorcurrent of transistor 504, minus that of transistor 503, charges thecapacitor 505, and the collector current of transistor 511 flows throughresistor 515 turning on transistor 516. With transistor 516 turned on,output transistor 701 is inhibited from turning on irrespective of thepresence of collector current in transistor 426. Capacitor 505 willcontinue charging via the excess collector current of transistors 503and 504 until it reaches a voltage equal to that on the base oftransistor 506. An instant later, the base-emitter junction oftransistor 506 becomes reverse-biased due to the reduced current oftransistor 510, and the base-emitter junction of transistor 507 becomesforward-biased, thus switching transistor 507 on and transistor 506 off.The switching of these two transistors is aided by the splitcollector-transistor 517. Collector current of transistor 502 at thispoint flows through transistor 507 and the current mirror formed aroundtransistor 509 is turned off. The addition of resistor 518 improves theturn-off characteristics of the mirror.

When transistors 507 and 506 switch states, immediately the base voltageof transistor 506 reduces by the voltage drop across resistor 514 to theoriginal reference voltage level. Capacitor 505 then starts to dischargevia the constant collector current of transistor 503. Also, the basedrive to transistor 516 discontinues, thus turning it off. If a validringing signal has been detected as discussed earlier, transistor 426will be supplying current to the output circuit means 700 so that whentransistor 516 turns off, the current in transistor 426 serves to gatetransistor 701 in the output circuit means 700 on. Thus, an inversion ofthe oscillator output voltage is achieved. Capacitor 505 then continuesto discharge through transistor 503 until its voltage drops to the levelof that on the base of transistor 506. At this time, transistor 507turns off and transistor 506 turns on again powering the current sourcesmirrored around transistor 509. The base of transistor 506 returns toits former voltage level, capacitor 505 starts charging, and theoscillation is repeated.

The frequency determining means 600 provides the option of having afrequency modulated type output signal by shifting the oscillatingfrequency of the oscillatory genertor means 500 during the negative halfcycles of the input ringing voltage. The circuit consists of transistors601 through 604 and resistors 605 through 610. The frequency shiftoption is enabled by connecting the base of transistor 604 throughresistor 610 and a switch such as 611 to line 102 in FIG. 2. Then on thenegative-going cycles of the input signal, transistor 604 will beforward-biased which shunts base current away from transistor 601turning it and transistor 603 off. This action allows transistor 602 tobecome part of the current mirror formed by transistor 509 causingcurrent to be shunted away from transistor 510 through resistor 608 andtransistor 602 to common. The result is a lower voltage appearing at thebase of transistor 506 which, in turn, has the effect of decreasing thevoltage to which capacitor 505 must charge to cause transistors 506 and507 to switch states. This, in turn, decreases the time capacitor 505charges and discharges and, thus, increases the frequency of theoscillator. For the remainder of the negative cycles of the inputsignal, transistor 602 will cause the collector current of transistor510 to be reduced, thus causing the increased oscillating frequency. Thevalue of resistor 608 is chosen to give an increased frequency that is a5/4th musical relationship to the original frequency, yielding apleasant sound. It should be noted that the duty cycles of bothfrequencies remain the same since the charge and discharge currents ofcapacitor 505 are unchanged. If the frequency shift option is notenabled, transistor 601 remains on keeping transistor 603 saturated. Asa result, transistor 602 remains turned off, no current is shunted fromtransistor 510 and the oscillator maintains the original frequency.

Although depicted in a specific embodiment in FIG. 3, the oscillatorygenerator means 500 with a frequency shift input may be depicted in amore general embodiment, as shown in FIG. 5. Included in the oscillatorygenerator means 500 are three current sources 520, 521 and 522, acomparator 523, a capacitor C and a resistor R. Current source 520 has acurrent of magnitude I and is on continuously. Current source 521 has acurrent of magnitude K₁ I and is turned on when the output of comparator523 is a logic 1. Current source 522 is also turned on when the outputof comparator 523 is a logic 1 and has a magnitude of K₂ I or K₃ I,depending on whether a frequency shift input signal is present. Thisrelationship becomes apparent from an examination of the truth tablealso shown in FIG. 5.

The comparator 523 has two input lines A and B and an output line D. Avoltage V_(ref) is coupled through resistor R to input line A of thecomparator. Current source 522 also connects between input line A and acommon reference potential such as ground. Capacitor C and currentsources 520 and 521 are connected between input line B and also thecommon reference potential. Output line D, which is a logic 1 if inputline A is greater than input line B and a logic 0 if input line A isless than input line B, connects the output of the comparator 523 tocurrent sources 521 and 522. Current source 522 also has the frequencyshift input signal connected thereto.

The capacitor C is discharged with the current I from current source 520and is charged with the current K₁ I from current source 521. Moreover,since K₁ I is a ratio of I, the net charging current ca be shown as (K₁-1)I. The three voltage threshold levels that can be across the resistorR and 0, RK₂ I, or RK₃ I, depending upon whether the frequency shiftsignal is present. Thus, the voltage that capacitor C must slew betweenthresholds is either 0 and RK₂ I or 0 and RK₃ I.

The time interval that capacitor C is charging when the output of thecapacitor 523 is at a logical 1 is: ##EQU1## Thus it can be seen by theabove that (1) the frequency and duty cycle are determined by the valuesof the resistor R, the capacitor C, and the ratio of the I and K₁ Icurrent sources, and (2) changing the frequency by changing R, C, or K₂will not change the duty cycle.

Operation of the oscillator is as follows assuming as a starting pointthe discharging of capacitor C. The voltage across the capacitor C andon the input line B is discharged by the current source 520 to below theV_(ref) voltage on input line A. This changes the output of comparator523 to a logic one. This logic one then turns on the current source 521having a current of K₁ I. A charging current of (K₁ -1)I then charges upthe capacitor C. The logic one from the comparator 523 also turns on thecurrent source 522 having a current of K₂ I or K₃ I which raises thevoltage on the A input line to a voltage represented by RK₂ I or RK₃ Iplus V_(ref), respectively. As the capacitor C charges up to the pointwhere input line B to the comparator becomes more positve than inputline A, the output of the comparator 523 goes to zero. Current source522 turns off and the voltage on input line A becomes V_(ref) again.Also, current source 521 turns off and current source 520 starts todischarge the capacitor C down from a voltage equal to RK₂ I plusV_(ref) or RK₃ I plus V_(ref) to less than the V_(ref) voltage at whichpoint the comparator output changes to the logic one state and theoscillation is repeated. It is seen, therefore, that the voltage swingof the capacitor is determined by the current K₂ I or K₃ I that isprovided by current source 522, and the rate which the capacitor C ischarged and discharged by the current sources 521 and 520, respectively.Thus, the period of oscillation is a function of the resistive andcapacitor values plus the ratio of the current sources (the K terms areall constant). Moreover, in that the current sources are interrelated,the current from the sources may vary over a large range withoutaffecting the frequency of operation so long as these ratios aremaintained. The effect of supply voltage variations on the oscillator'sfrequency of operation is thereby minimal.

The output circuit means 700 for the tone ringer is formed by transitors701, 702 and resistor 703 connected in an equivalent silicon-controlledrectifier (SCR) combination. As discussed previously, when a validringing signal is recongnized by the sensing means 400, a continuouscurrent is supplied from the collector of transistor 426 to the outputcircuit means 700. Since by this time the oscillatory generator means500 is running, transistor 516 will be switching on and off at thefrequency and duty cycle of the oscillatory generator means. Whentransistor 516 is on, the gate current from transistor 426 will passthrough its collector and, therefore, the equivalent SCR will be off.Conversely, when transistor 516 is off, the SCR will be on via currentsupplied by transistor 426.

The complete output circuit means 700 includes the transducer connectionwhich obtains drive power through rectifier diode 205. Some smoothing ofthe rectified input signal voltage is provided by capacitor 705 so thatthe resulting voltage applied to the transducer is amplitude modulatedwith a modification depth of about 40%; the amplitude-modulated inputvoltage thus provides an envelope to the pulse which drives thetransducer. Resistors 706 through 709 form a volume control for the toneringer. Diode 710 passes the reverse EMF generated by the decayingvoltage through the inductance of the transducer 300.

Although both a specific and general embodiment of the invention havebeen shown and described, it will be understood that they are butillustrative and various modifications may be made therein withoutdeparting from the scope and spirit of this invention as defined in theappended claims.

We claim:
 1. An oscillatory signal generator circuit comprising incombination:a capacitor (C); means (V_(ref), R, 522) for providing afirst voltage reference potential and a second voltage referencepotential for determining the charging and discharging voltage range ofthe capacitor, and for causing and discharging voltage range of thecapacitor, and for causing operation of the generator circuit at a firstfrequency; a plurality of interrelated constant current sourcesincluding a first current source (522) and wherein the means include incombination a resistor (R) and and voltage source (V_(ref)) and thefirst current source for providing the first voltage referencepotential, and the voltage source providing the second voltage referencepotential, the plurality of constant current sources further including asecond current source (520) for discharging the capacitor to the secondvoltage reference potential and a third current source (521) forcharging the capacitor to the first voltage reference potential; and acomparison means (523) for comparing the voltage on the capacitor withthe first and second voltage reference potentials, the comparison meansprovidng an input signal to the first current source and to the thirdcurrent source upon the capacitor voltage going outside of the voltagerange defined by the first and second voltage reference potentials, thecapacitor, the second current source and the third current source beingcommonly connected to a third reference potential and to a first inputof the comparison means, a second input of the comparison means havingthe voltage source coupled thereto through the resistor, and one side ofthe first current source connected thereto, the other side of the firstcurrent source being connected to the third reference potential.
 2. Thegenerator circuit of claim 1 wherein the frequency of oscillation isdetermined by the resistor value and capacitor value and the ratio ofthe third current source to the second current source, the third currentsource being a ratio of the second current source, and the first currentsource being a ratio of the second current source, the interrelating ofthe current sources providing a frequency insensitivity to supplyvoltage variations.
 3. The generator circuit of claim 2 wherein afrequency control signal provides an input to the first current source.4. The generator circuit of claim 3 wherein the first current source inresponse to the frequency control signal shifts the first voltagereference potential to a different magnitude for changing the voltagerange over which the capacitor must charge and discharge, and forcausing the oscillatory signal generator circuit to operate at a secondfrequency.
 5. An oscillatory signal generator circuit comprising incombination:a capacitor (C); a first current source (521) and a secondcurrent source (520), the first current source providing a chargecurrent to the capacitor and the second current source providing adischarge current to the capacitor; a third current source (522) fordetermining the charging and discharging voltage range on the capacitor,the voltage range being between a first voltage reference potentialdetermined by the combination of a voltage source (V_(ref)), a resistorand the current output of the third current source, and a second voltagereference potential determined by the voltage source; and a comparisonmeans (523) for comparing the voltage on the capacitor with the firstand second voltage reference potentials, the comparison means providingan output signal indication to the first current source and to the thirdcurrent source upon the capacitor voltage going outside the voltagerange by the first and second voltage reference potentials, thecapacitor, the second current source and the third current source beingcommonly connected to a third reference potential and to a first inputof the comparison means, a second input of the comparison means havingthe voltage source coupled thereto through the resistor, and one side ofthe first current source connected thereto, the other side of the firstcurrent source being connected to the third reference potential.